1. Field of the Invention
This invention relates to a spark plug including a central electrode and an earth electrode having a chip including a noble metal to have a gap between the chip and the central electrode and to a method of producing the spark plug by laser welding.
2. Description of the Prior Art
The use of a noble metal chip at the tip of the earth electrode of a spark plug to form a spark gap between the chip and the central electrode provides a long effective life of the spark plug and/or reduces misfiring.
U.S. Pat. No. 6,215,235 discloses a spark plug in which a noble metallic firing chip is bonded on the opposing surface of either a central electrode or an earth electrode by laser welding. In this spark plug, the base electrode member (the central electrode or the earth electrode) has a slender tip with a noble metal chip welded thereto by laser to provide a long effective life and excellent ignitability.
U.S. Pat. No. 5,811,915 discloses a spark plug in which a noble metallic firing chip is sunk in the base electrode member, and then, the swelled portion around the chip is welded to the chip.
In these spark plugs, the noble metallic firing chips on the central electrodes are fixed with practically sufficient strengths. On the other hand, the noble metallic firing chips on the earth electrodes are fixed with practically insufficient strengths. This is caused by difference in the fixing condition. That is, the noble metallic firing chip on the central electrode is fixed near the body of the spark plug, so that thermal stress at the fused junction layer connecting the noble metallic firing chip to the central electrode is relatively low. On the other hand, the noble metallic firing chip on the earth electrode is fixed away from the body of the spark plug. In other words, the fused junction layer connecting the noble metal chip to the earth electrode is more exposed to the combustion chamber. Accordingly, the fused junction layer on the earth electrode is subjected to relatively high thermal stress. This may result in development of cracks and finally result in disconnection of the noble metallic chip from the earth electrode.
The aim of the present invention is to provide a superior spark plug and a superior method of producing the same.
According to the present invention, a first aspect of the present invention provides a spark plug comprising:
a tubular housing;
a central electrode supported by said tubular housing in said tubular housing with electrical insulation therebetween;
an earth electrode extending from one end of said tubular housing;
a chip, arranged at an end surface of said earth electrode to face said central electrode, for providing a spark gap between said central electrode and said chip, said chip including a novel metal; and
a fused junction layer between said earth electrode and said chip including components of said chip and said earth electrode to fix said chip to said earth electrode, wherein a cross-sectional area of said chip at a tip thereof on the opposite side of said fused junction layer is not less than 0.12 mm2 and not more than 1.15 mm2, and a length from said end surface to a top surface of said tip is not less than 0.3 mm2 and not more than 1.5 mm2, and wherein said fused junction layer has substantially a conical outer surface continuously connecting a peripheral outer surface of said chip to said end surface of said earth electrode with a radius on a sectional plane along an axis of said chip.
According to the present invention, a second aspect of the present invention provides a spark plug based on the first aspect, wherein if it is assumed that a maximum width of said chip on a sectional plane along said axis is D and that said radius is R, D/4xe2x89xa6Rxe2x89xa63D/4.
According to the present invention, a third aspect of the present invention provides a spark plug based on the first aspect, wherein said chip mainly includes Ir and further includes at least one of Rh, Pt, Ni, W, Pd, Ru, and Os.
According to the present invention, a fourth aspect of the present invention provides a spark plug based on the third aspect, wherein said chip mainly includes Ir and further includes at least one of Rh of lower than or equal to 50% by weight, Pt of lower than or equal to 50% by weight, Ni of lower than or equal to 40% by weight, W of lower than or equal to 30% by weight, Pd of lower than or equal to 40% by weight, Ru of lower than or equal to 30% by weight, and Os of lower than or equal to 20% by weight.
According to the present invention, a fifth aspect of the present invention provides a spark plug based on the first aspect, wherein said chip mainly includes Pt and further includes at least one of Ir, Ni, Rh, W, Pd, Ru, and Os.
According to the present invention, a sixth aspect of the present invention provides a spark plug based on the third aspect, wherein said chip mainly includes Pt and further includes at least one of Ir of lower than or equal to 50% by weight, Ni of lower than or equal to 40% by weight, Rh of lower than or equal to 50% by weight, W of lower than or equal to 30% by weight, Pd of lower than or equal to 40% by weight, Ru of lower than or equal to 30% by weight, and Os of lower than or equal to 20% by weight.
According to the present invention, a seventh aspect of the present invention provides a spark plug based on the third aspect, wherein said fused junction layer includes said component of said chip of not less than 35% by weight and not more than 80% by weight.
According to the present invention, an eighth aspect of the present invention provides a spark plug based on the fifth aspect, wherein said fused junction layer includes said component of said chip of not less than 35% by weight and not more than 80% by weight.
According to the present invention, a ninth aspect of the present invention provides a method of producing a spark plug including a tubular housing, a central electrode supported by said tubular housing in said tubular housing with electrical insulation therebetween, and an earth electrode extending from one end of said tubular housing, comprising the steps of:
placing said chip including a noble metal on a surface of a tip of said earth electrode with contact between an end surface of said chip and said surface; and
welding said chip to said surface by applying a laser beam toward a corner made between said surface and a side surface neighboring said end surface of said chip at an inclined angle to said end surface and said side surface.